Magnetic focusing structure for traveling wave tubes



2 Sheets-Sheet 1 INVENTOR K. M. POOLE Arromv r K. M. POOLE INN & mw aMAGNETIC FOCUSING STRUCTURE FOR TRAVELING WAVE TUBES Nov. 3, 1964 FiledMarch 14, 1961 Nov. 3, 1964 K. M. POOLE MAGNETIC FOQUSING STRUCTURE FORTRAVELING WAVE TUBES Filed March 14. 1951 2 Sheets-Sheet 2 mm mm mP M lK m n .1/ U v ow mw u n z l I l 1;. w n k WW -.|i| 7 l I 2| I I I l lIIL l|| I! W m m b kw J): m Mm om Q X N.Q\|-\ United States Patent M3,155,866 MAGNETEC FGCUSING STRUCTURE FOR TRAVELING WAVE TUBES KennethM. Poole, New Providence, Ni, assignor to Bell Telephone Laboratories,Incorporated, New York,

N.Y., a corporation of New York Filed Mar. 14, 1961, Ser. No. 95,724 3Claims. (Cl. 3153.5)

This invention relates to traveling wave tubes and, more particularly,to a magnetic electron beam focusing arrangement for such tubes.

In a copending application of C. C. Cutler, Serial No. 168,202, filedJune 15, 1950, now US. Patent No. 3,005,- 126, issued October 17, 1961,there is described a beam focusing arrangement for traveling wave tubeswherein the electron beam is first formed in a region free of magneticflux, and then is made to encounter abruptly a magnetic field havingradially directed flux which imparts a spin to the electrons in thebeam. The beam thereafter is immersed in a longitudinal magnetic field.Such a focusing arrangement has several advantages. For example, where ahigh density electron beam is desirable, the above-described systempermits the use of much smaller magnetic field than the ordinaryconfined flow type of focusing. This, in turn, means that less magnetweight is required to focus a beam of a given current density. Areduction in magnet Weight is always desirable, for economic reasons aswell as for reasons of space and portability. In the case of airborne orspace applications, however, a minimum magnet weight is an absolutenecessity.

In the aforementioned and similar focusing systems, ideally the electrongun is completely shielded from the magnetic field. In practice,however, some stray magnetic flux enters the electron gun region throughthe stem end of the traveling wave Where there is no magnetic shield.This stray magnetic flux, the direction of which is counter to that ofthe main focusing field, produces undesirable perturbations in the beamwhich tend to degrade the focusing and reduce the usable beam currentdensity. Much of the stray flux may be eliminated by the use ofadditional shielding; however, additional shielding means increasedweight, which, as pointed out in the foregoing, is intolerable inairborne and space applications.

It is an object of this invention to eliminate the deleterious effectsof stray magnetic flux in the gun region of a shielded gun type focusingsystem.

It is another object of this invention to eliminate the effects of suchstray magnetic flux with no increase in the weight of the focusingsystem.

These and other objects of the present invention are achieved in anillustrative embodiment thereof which comprises an elongated, permanentmagnet having a longitudinally extending centrally located bore forestablishing a longitudinal magnetic focusing field in the bore. At eachend of the magnet is an apertured pole-piece. A traveling wave tube ismounted Within the bore of the permanent magnet with the cathode end ofthe tube situated outside of the magnetic structure. A magneticshielding member extending from the pole-piece adjacent the cathode endof the tube surrounds the cathode end and, as taught in theaforementioned Cutler application, acts to shield the electron gun fromthe magnetic field. The portion of the tube envelope which surrounds theelectron gun, hereinafter referred to as the bulb, is made of a materialof high permeability and is situated within the aperture of the adjacentmagnetic pole-piece and extends outward therefrom, being surrounded bythe magnetic shield.

It is one feature of the present invention that the outer diameter ofthat portion of the bulb which is located principles of the presentinvention.

3,155,866 Patented Nov. 3, 1964 within the aperture in the pole-piece issmaller than the diameter of the pole-piece aperture so that there isprovided between the pole-piece and the bulb an air gap forming a highreluctance path for the magnetic flux.

It is another feature of the present invention that the bulb has anintegral external flange remote from the portion located within thepole-piece, the periphery of the flange being a slip fit within themagnetic shield whereby there is formed a low reluctance path for themagnetic flux.

These and other features of the present invention will be more readilyapparent from the following detailed description, in conjunction withthe accompanying drawings, in which:

FIG. 1 is a sectioned view of a traveling wave tube and focusingmagnetic assembly embodying the principles of the present invention; and

FIG. 2 is a diagrammatic view of the electron gun region of the tube ofFIG. 1.

Turning now to FIG. 1, there is shown in section a traveling wave tubeand magnet assembly embodying the The assembly cornprises a permanentmagnet 11 having a longitudinally extending, centrally located bore 12.At either end of the magnet 11 are apertured pole-pieces 13 and 14. Atraveling wave tube 16 having a bulb portion 17 of Kovar or othersuitable material having high magnetic permeability and an elongatedenvelope portion 18 of glass ismounted in the magnet assembly with theglass portion 18 extending through the bore 12. A plurality of pins 19serve to fix the envelope portion 13 centrally within the bore.

Between the envelope portion 18 and the wall of bore 12 are a pluralityof magnetic field straighteners 21 which are advantageously of the typedisclosed and claimed in United States Patent 2,942,141 of C. C. Cutler.explained in that patent, the field straighteners 21 act to minimizetransverse components of the magnetic field, \/hich produce undesirableperturbation of the electron eam.

Within the envelope portion 13 of tube 16 is a slow wave circuit 22. Asshown here circuit 22 is a helix, but it may take any one of a number offorms known to workers in the art. A helix connector 2%) is locatedwithin an aperture 25 in bulb 1'7 and enables the application of avoltage to circuit 22. Energy to be amplified is applied to circuit 22by means of an input Wave guide 23, and amplified energy is abstractedby means of an output Wave guide 24. The electron beam, after passagethrough the circuit 22, is collected by a collector 26 in a manner wellknown in the art.

Pole-piece 13 has an aperture 27 within which is located a portion ofthe bulb 17. In accordance with the present invention and for reasonswhich will be discussed more fully hereinafter, the diameter of aperture2'7 is made larger than the other diameter of that portion of bulb 17within the aperture. A magnetic shield 28 extends from pole-piece 13 andsurrounds bulb 17 over a major portion or its length. I

Within bulb 17 are mounted an electron emissive cathode 23, a beamforming electrode 31, and an accelerating anode 32 which coast toproduce an electron beam. For clarity, the mounting arrangement andelectrical leads to these elements have been omitted, and it is to beunderstood that they may take any one of a number of forms well known inthe art. In accordance with the present invention, bulb 17 has formedthereon a flange 33 having a machined surface 34 the diameter of whichis such as to form a slip fit with the inside diameter of shield 28. Aplurality of tabs 36, which are welded to shield 28 bear against flange33 and act to prevent translational movement of the tube 16.

As is' When the magnet 11 and the tube 16 are assembled, magnet 11produces an axial magnetic focusing field within tube 16, as indicatedby the arrow H. In addition, an external magnetic field, as indicated bythe arrows H, is also produced. Because the shield 28 is open at itsend, some of this external field H enters the stem end of tube 16 andthreads the cathode, tending to produce undesirable perturbations in thebeam.

For an understanding of the present invention and how it counteracts theeffect of the field H in the cathode region, reference should now bemade to FIG. 2. In FIG. 2, the relationship of pole-piece 13 and bulb 17in accordance with the present invention is shown. For clarity, thevarious electrodes of the electron gun have been omitted, although theoutline of the cathode 29 is shown in dotted lines. As is readilyapparent from FIG. 2, the magnetic focusing field H passes intoconnector 26 and then into bulb 17, which, as was pointed outheretofore, is of a material of high permeability such as Kovar. Withsuch an arrangement, the requisite radial components of magnetic fluxfor producing a spiraling motion to the electrons, as taught in theaforementioned Cutler application, is produced. Because the diameter ofaperture 27 in pole-piece 13 is greater than the diameter of thatportion of bulb 17 within the aperture, a high reluctance air gap isproduced and the flux H, instead of crossing this air gap, passesthrough the bulb 17 to the flange 33 where a low reluctance path isformed between machined surface 34 and shield 28. Because the lowreluctance path is toward the end of cathode 29 remote from its emissivesurface, a portion of the flux H tends to pass through the cathoderegion, as indicated by the arrows, instead of, as in the case of priorart devices, passing directly to pole-piece 13. This portion of flux Hwhich passes through the cathode region is opposite in direction to thestray flux H in the cathode region, as seen by the arrows in FIG. 2, andtends to buck it out. By proper choice of materials and dimensions, suchas the size of the air gap formed in aperture 27, the amount of flux Hin the cathode region can be made sutficient to cancel the flux H in thecathode region so that the electron beam in fact remains unaffected byany magnetic field, until it encounters the radial field in connector29. In practice it has been found that with the arrangement of thepresent invention, as discussed in the foregoing, the advantages of thefocusing arrangement of the aforementioned Cutler application are fullyrealized without the necessity of additional shielding or morecomplicated structure and the attendant, highly undesirable, increase inweight.

It is to be understood that the foregoing illustrative embodiment is forthe purpose of illustrating the principles of the present inventiononly. Other embodiments of these principles may readily occur to workersin the art without departing from the spirit and scope of the presentinvention.

What is claimed is:

1. In combination a traveling wave tube and a focusing structuretherefor, said traveling wave tube having a cathode having an electronemissive surface and an envelope comprising an elongated portion and abulb portion of magnetic material surrounding the cathode, said focusingstructure comprising magnet means and pole-pieces for producing amagnetic focusing field axially of said elongated portion of saidenvelope and a magnetic shield member extending from one of saidpole-pieces and surrounding said bulb portion over at least a portion ofits length, and means for establishing a low reluctance flux pathbetween said bulb and said shield comprising a flange on said bulbremote from the emissive surface of said cathode, said flange being incontact with said shield.

2. In combination a traveling wave tube and a focusing structuretherefor, said traveling wave tube having a cathode having an electronemissive surface and an envelope comprising an elongated portion and abulb portion sur rounding the cathode, said focusing structurecomprising magnet means coextensive with said elongated portion of saidenvelope and apertured pole-pieces at either end of said magnet meansfor producing a magnetic focusing field axially of said elongatedportion, a magnetic shield member extending from one of said pole-piecesadjacent said bulb portion and surrounding said bulb portion over atleast a portion of its length, the aperture in said one pole-piece beingof greater diameter than that portion of the bulb adjacent saidpole-piece whereby a high reluctance air gap is formed between saidpole-piece and said bulb, and means establishing a low reluctance fluxpath between said bulb and said shield comprising a portion of said bulbremote from the emissive surface of said cathode in contact with saidshield.

3. The combination as claimed in claim 2, wherein said bulb portion isof magnetic material.

References Cited in the file of this patent UNITED STATES PATENTS2,707,758 Wang May 3, 1955 2,774,006 Field et al. Dec. 11, 19562,828,434 Klein et al. Mar. 25, 1958 2,844,750 Veith et al July 22, 19582,871,395 Ciotfi Ian. 27, 1959

1. IN COMBINATION A TRAVELING WAVE TUBE AND A FOCUSING STRUCTURETHEREFOR, SAID TRAVELING WAVE TUBE HAVING A CATHODE HAVING AN ELECTRONEMISSIVE SURFACE AND AN ENVELOPE COMPRISING AN ELONGATED PORTION AND ABULB PORTION OF MAGNETIC MATERIAL SURROUNDING THE CATHODE, SAID FOCUSINGSTRUCTURE COMPRISING MAGNET MEANS AND POLE-PIECES FOR PRODUCING AMAGNETIC FOCUSING FIELD AXIALLY OF SAID ELONGATED PORTION OF SAIDENVELOPE AND A MAGNETIC SHIELD